Among the well known nondestructive testing techniques are the techniques of spectroreflectometry and spectroscopic ellipsometry, which measure reflectance data by reflecting electromagnetic radiation from a sample. In spectroscopic ellipsometry, an incident radiation beam having a known polarization state reflects from a sample (generally at high incidence angle), and the polarization of the reflected radiation is analyzed to determine properties of the sample. Since the incident radiation includes multiple frequency components, a spectrum of measured data (including data for incident radiation of each of at least two frequencies) can be measured. Typically, the polarization of the incident beam has a time-varying characteristic (produced, for example, by passing the incident beam through a mechanically rotating polarizer), and/or the means for analyzing the reflected radiation has a time-varying characteristic (for example, it may include a mechanically rotating analyzer). Examples of spectroscopic ellipsometry systems are described in U.S. Pat. No. 5,329,357, issued Jul. 12, 1994 to Bernoux, et al., and U.S. Pat. No. 5,166,752, issued Nov. 24, 1992 to Spanier, et al.
In the technique of spectroreflectometry an incident radiation beam reflects from a sample, and the intensity of the reflected radiation is analyzed to determine properties of the sample. The incident radiation includes multiple frequency components (or is monochromatic with a time-varying frequency), so that a spectrum of measured data (known as a reflectance spectrum or relative reflectance spectrum) including data regarding reflected intensity of incident radiation having each of at least two frequencies is measured. Systems for spectroreflectometry are described in U.S. Pat. No. 5,241,366 issued Aug. 31, 1993 to Bevis et al., and U.S. Pat. No. 4,645,349, issued Feb. 24, 1987 to Tabata, and the following U.S. patent applications assigned to the assignee of the present invention: U.S. Ser. No. 07/899,666, filed Jun. 16, 1992 (abstract published on Apr. 26, 1994 as the abstract of U.S. Pat. No. 5,306,916), and pending U.S. Ser. No. 08/218,975, filed Mar. 28, 1994.
Reflectance data (measured by spectroscopic ellipsometry, spectroreflectometry, or other reflection techniques) are useful for a variety of purposes. The thickness of various coatings (either single layer or multiple layer) on a wafer can be determined from spectroscopic ellipsometry data (indicative of the polarization of radiation reflected from the sample in response to incident radiation having known polarization state), or a reflectance spectrum or relative reflectance spectrum.
The reflectance of a sample (or sample layer) at a single wavelength can be extracted from a reflectance or relative reflectance spectrum. This is useful where the reflectance of photoresist coated wafers at the wavelength of a lithographic exposure tool must be found to determine proper exposure levels for the wafers, or to optimize the thickness of the resist to minimize reflectance of the entire coating stack.
The refractive index of a coating on a sample (or layer thereof) can also be determined by analysis of spectroscopic ellipsometry data (indicative of the polarization of radiation reflected from the sample, in response to incident radiation having known polarization state) or an accurately measured reflectance spectrum.
It would be useful for a variety of industrial applications to determine the thickness of a very small region of a very thin film (less than 30 angstroms in thickness) on a substrate from reflectance measurements (with sub-angstrom measurement repeatability) of the sample (e.g., where the sample is a semiconductor wafer and the very thin film is coated on a silicon substrate of the wafer). It would also be useful for a variety of industrial applications to obtain reflectance measurements using a single measurement system, and then analyze the measured data to determine the refractive index and thickness of a layer of a sample, where the layer has unknown thickness in a broad range from more than 10 microns to less than 10 angstroms.
It would also be useful to obtain reflectance measurements using a single measurement system, and then analyze the measured data to determine the refractive index and thickness of any selected layer of a multiple layer stack (where each layer has unknown thickness in a range from more than 10 microns to less than 10 angstroms). Such multiple layer stacks are often produced during the manufacture of semiconductor integrated circuits, with the stacks including various combinations of material such as SiO.sub.2, Si.sub.3 N.sub.4, TiN, Poly-Si, and a-Si.
Because of the tight tolerance requirements typically required in the semiconductor arts, an extremely accurate method and apparatus (e.g., having sub-angstrom repeatability) is needed for determining film thickness and refractive index measurements from reflectance data from a very small, and preferably compact region (e.g., a microscopically small region of size less than 40 micron.times.40 micron) of a wafer. However, it had not been known how to accomplish this using an ellipsometer with all-reflective optics (for use with broadband UV radiation). Conventional ellipsometers had employed transmissive optics to direct a beam at a sample, either with relatively high incidence angles (angles substantially greater than the zero degree incidence angle of "normally" incident radiation at a sample) as in above-cited U.S. Pat. No. 5,166,752, or with low incidence angle (normal or nearly normal incidence at the sample). The inventors have recognized that such transmissive optics are unsuitable for use with broadband radiation of ultraviolet (or UV to near infrared) wavelengths, and have also recognized that beams of such radiation incident on reflective ellipsometer components with high incidence angles undesirably undergo a large change in polarization upon reflection from each such reflective component. The inventors have also recognized that the change in beam polarization upon reflection from each optical component of an ellipsometer should be small relative to the polarization change (due to specific properties of the sample itself) occurring on reflection from the sample, and that such small polarization changes can be achieved by reflecting an ellipsometer beam from optical components of an ellipsometer only at small incidence angles (where the ellipsometer reflectively focuses the beam to a small, compact spot on the sample, with rays of the beam incident at the sample at a substantial range of high incidence angles).
Until the present invention, it had not been known how to meet the needs set forth in all three preceding paragraphs, and avoid the described limitations of the prior art set forth in these three preceding paragraphs.